It is known that chromium-based catalyst systems with diphosphine ligands catalyse the selective conversion of ethylene to 1-hexene and/or 1-octene depending on the reaction conditions and choice of ligand structure. In particular, the nature and position of any substituents on the aryl rings connected to the phosphines are crucial influences on the selectivity split between 1-hexene and 1-octene. Of particular interest to industry are catalysts for ethylene tetramerisation, as these catalysts are relatively rare. Octene is a valuable co-monomer for the production of high performance linear low density polyethylenes and elastomers, and few selective on-purpose routes to this chemical are known in industry. By comparison, catalysts for ethylene trimerisation are relatively common, and are used industrially by several companies. By tetramerisation it is meant that at least 30% 1-octene is produced in the process. By trimerisation it is meant that more than 70% 1-hexene is produced.
Non-limiting examples of selective ethylene oligomerisation catalyst systems include the ubiquitous Cr/bis(phosphino)amine (i.e. ‘PNP’) systems, particularly of the type (Ar1)(Ar2)PN(R)P(Ar3)(Ar4), where Ar1 to Ar4 are aryl groups such as phenyl and R is a hydrocarbyl or a heterohydrocarbyl group, beginning with PNP ligands containing no substituents on the phenyl rings bonded to the P-atoms (e.g. as described in WO 2004/056479) and those with m- or p-methoxy groups on the phenyl rings (e.g. as described in WO 2004/056480). In addition to this, PNP systems containing o-fluoro groups on the phenyl rings are described in US 2008/0242811 and US 2010/0081777, and PNP systems bearing pendant donor atoms on the nitrogen linker are described in WO 2007/088329. Multi-site PNP ligands are discussed in US 2008/0027188. In addition to the Cr/PNP systems, chromium systems bearing N,N-bidentate ligands (e.g. as described in US 2006/0247399) can be used. PNP ligands with alkylamine or phosphinoamine groups bonded to one of the PNP phosphines (i.e. ‘PNPNH’ and ‘PNPNP’ ligands) are described in WO 2009/006979. Finally, carbon bridged diphosphine (i.e. ‘PCCP’ ligands) are described in WO 2008/088178 and WO 2009/022770.
Related ethylene trimerisation catalysts with high selectivity for 1-hexene can be obtained by using PNP ligands with ortho-methoxy or ortho-alkyl substituents on the phenyl rings bonded to the P-atoms (e.g. as described in WO2002/04119, WO2004/056477 and WO2010/034101).
The above catalyst systems suffer from a number of shortcomings. These include low catalyst activity and high polymer co-product formation when operated at elevated temperatures, especially above 80° C., and high selectivity towards heavy oligomers (C10 to C30+ olefins). These problems are especially evident for tetramerisation catalysts, where the challenge of obtaining good catalyst performance together with good selectivity towards 1-octene at high reaction temperatures is severe.
In a recent review article describing catalyst systems for ethylene tetramerisation, van Leeuwen et al (Coordination Chemistry Reviews, 255, (2011), 1499-1517) have discussed the problems associated with elevated reaction temperatures. They state that: “In general the selective ethylene tetramerisation experiments are performed in the temperature range 40-60° C. Various studies on both semi-batch and continuous miniplant have shown a strong dependency of the reaction temperature on the activity and selectivity of the Cr(III)/Ph2N(R)PPh2/MAO catalytic system. High reaction temperatures (>60° C.) significantly reduced the catalyst productivity as compared to reactions performed at lower temperature under the same ethylene pressure. Consequently catalyst decomposition with increasing temperature is probably the main reason for lower productivities at high temperatures . . . ”
When carrying out a process for tetramerisation of ethylene, the aim is to choose a catalyst system and adjust process conditions in order to produce the maximum amount of 1-octene, as opposed to trimerisation processes where catalysts and process conditions are adjusted to produce the maximum amount of 1-hexene. 1-Hexene is also typically co-produced in a tetramerisation process and it is well known in the art of the invention that higher temperatures shift the selectivity from 1-octene towards 1-hexene. Apart from 1-octene and 1-hexene, which are typically the targeted products in a selective oligomerisation process, various other co-products are formed in tetramerisation reactions, notably heavy (C10+) oligomers predominantly formed by secondary reactions of 1-hexene or 1-octene with ethylene. Tetramerisation catalysts which minimize the formation of these unwanted co-products are highly desirable.
Furthermore, the formation of a high molecular weight polymer co-product by the Cr-based ethylene tetramerisation catalyst may present a major technical challenge when commercialising an ethylene tetramerisation process as polymer fouling reduces plant run time and necessitates shut-downs due to blockages and difficult temperature control. When running tetramerisation processes at reaction temperatures in the range of 40 to 80° C., the polymer precipitates out of solution in the reactor, which brings risk to the process due to the possibility of reactor or downstream equipment fouling.
Consequently, new catalyst systems which can operate with good rates, low polymer formation, good 1-octene to 1-hexene ratios and reduced selectivity to heavy oligomers are highly desirable. Such catalysts would be useful at oligomerisation temperatures of 40 to 80° C., by reducing the amount of unwanted co-products formed, including polyethylene and heavy oligomers. Alternatively, they could be useful at higher oligomerisation reaction temperatures, where the polymer co-product remains in solution, but where catalyst stability and adequate selectivity to 1-octene are the greatest challenges.